The present disclosure generally relates to processing of materials for growth of crystals. More particularly, the present disclosure provides a mineralizer suitable for use as a raw material for crystal growth of a group III metal nitride crystal by an ammonoacidic technique, but there can be others. In other embodiments, the present disclosure provides methods suitable for synthesis of crystalline nitride materials, but it would be recognized that other crystals and materials can also be processed. Such crystals and materials include, but are not limited to, GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
Gallium nitride containing crystalline materials serve as substrates for manufacture of conventional optoelectronic devices, such as blue light emitting diodes and lasers. Such optoelectronic devices have been commonly manufactured on sapphire or silicon carbide substrates that differ in composition from the deposited nitride layers. In the conventional Metal-Organic Chemical Vapor Deposition (MOCVD) method, deposition of GaN is performed from ammonia and organometallic compounds in the gas phase. Although successful, conventional growth rates achieved make it difficult to provide a bulk layer of GaN material. Additionally, dislocation densities are also high and lead to poorer optoelectronic device performance.
Growth of nitride crystals by ammonothermal synthesis has been proposed. Ammonothermal crystal growth methods are expected to be scalable, as described by Dwilinski, et al, J. Crystal Growth 310, 3911 (2008), by Ehrentraut, et al., J. Crystal Growth 305, 204 (2007)], by D'Evelyn, et al. J. Crystal Growth 300, 11 (2007), and by Wang, et al., Crystal Growth & Design 6, 1227 (2006). The ammonothermal method generally requires a mineralizer, which chemically reacts with a polycrystalline source material to form a soluble intermediate that is transported in a supercritical fluid and is then recrystallized onto seed crystals. An ongoing challenge of ammonothermally-grown GaN crystals is a significant level of impurities, which cause the crystals to be colored, e.g., yellowish, greenish, grayish, or brownish. The residual impurities may cause optical absorption in light emitting diodes fabricated on such substrates, negatively impacting efficiency, and may also degrade the electrical conductivity and/or generate stresses within the crystals. One potential source of the impurities is the mineralizer.
A number of mineralizers have been proposed for ammonothermal growth of crystalline group III nitrides. These include alkali metals; alkali imide, imido-amide, amide, nitride, hydride, or azide; an alkaline earth metal, imide, imido-amide, amide, nitride, hydride, or azide; ammonium halide, a group III metal halide, or a reaction product between a group III metal, ammonia, and hydrogen halide. Most of these mineralizers are highly hygroscopic and/or moisture sensitive, with the consequence that it is rather difficult to achieve low levels of oxygen impurity. For ammonobasic mineralizer chemistry, Dwilinski, et al. (U.S. Pat. No. 7,364,619) proposed the use of azides, which are commercially available and are less hygroscopic and therefore easier to purify than the corresponding amides or nitrides. However, azides have the disadvantage of being chemically unstable and may decompose to form excess nitrogen gas under typical ammonothermal conditions. For ammonoacidic chemistry, ammonium chloride and ammonium fluoride are commercially available, with purity specifications above 99.99% on a trace metals basis (that is, the impurity levels of oxygen and moisture are not specified). Ammonoacidic mineralizers, including mineralizers containing fluoride, may offer certain advantages over ammonobasic mineralizers. Stepin, et al., (Poluch. Anal. Vestchestv. Osoboi Chist., 5th, 91-94 (1978)) suggested forming NH4Cl from HCl and NH3, and Naumova, et al. (Zh. Prikh. Khim. 52, 249 (1979)) suggested purifying NH4Cl by sublimation. However, to the best of our knowledge, none of these authors specified the oxygen impurity levels that were achievable by these methods.
Mikawa et al. (U.S. Application Publication No. 2011/0268645) disclosed formation of ultrapure ammonium halides by reaction of ultrapure hydrogen halide with ultrapure ammonia and their use as a mineralizer for ammonothermal gallium nitride crystal growth. However, the methods disclosed by Mikawa et al. are not well suited for working with a condensable hydrogen halide such as HF, useful for synthesizing fluoride-containing mineralizers.
What is needed is a method for low-cost manufacturing of fluoride-containing mineralizers that are suitable for crystal growth of bulk gallium nitride crystals and do not contribute to impurities in the bulk crystals.